Cell Specialisation (HL IB Biology)

• In complex multicellular organisms eukaryotic  cells become specialised for specific functions. This can also be referred to as the division of labour
• The process occurs after fertilisation to allow development of different tissues within the embryo
• Specialisation enables the cells in these tissues to function more efficiently as they develop specific adaptations for their role. The development of these distinct specialised cells occurs by differentiation
• These specialised eukaryotic cells have specific adaptations to help them carry out their functions
• For example, the structure of a cell is adapted to help it carry out its function (this is why specialised eukaryotic cells can look extremely different from each other)
  
  • Structural adaptations include:
    • The shape of the cell
    • The organelles the cell contains (or doesn’t contain)
      • For example: Cells that make large amounts of proteins will be adapted for this function by containing many ribosomes (the organelle responsible for protein production)

• During the differentiation process, cell sizes can vary drastically
• Size is a feature of adaptation which means that cells require different dimensions to carry out their jobs efficiently

Cell Specialisation Diagram

Cells that have differentiated are specialised cells; they come in all shapes and sizes

How differentiation makes cells adapted to their function

• Red blood cells are small to allow movement through narrow capillaries
• Active white blood cells are larger than inactive white blood cells to allow space for rER and Golgi apparatus to allow protein (antibody) synthesis
• Sperm cells are long to allow for movement towards the egg cell, they also have narrow streamlined heads to reduce resistance to reaching the egg cell
• An egg cell body has the largest volume of all cells to allow for stored food reserves.
• A nerve cell has a large cell body to allow for protein synthesis to maintain the structure of the long axon which is required for rapid delivery of impulses around the nervous system
• Muscle cells are larger than normal cells, length and diameter is designed to exert force during muscle contraction

Surface area to volume ratio

• For cells to survive, metabolic reactions must be occurring; these reactions rely on materials being constantly exchanged across the plasma membrane at the cell's surface
• The metabolic requirements of a cell will vary depending on the volume or mass of cytoplasm (as this is where the reactions take place); a cell with a larger volume will have higher metabolic requirements, and vice versa
• As cells increase in size their surface area to volume ratio (SA:V) decreases as there is less surface area in relation to the volume of the organism
• So, an increase in volume will increase a cell's metabolic requirements, but its ability to carry out exchange with its environment does not increase at the same rate

Constraints on cell size

• Single-celled organisms have a high SA:V ratio; this means that they can survive by exchanging substances with their environment by simple diffusion at the cell surface
  • Their metabolic requirements are relatively low
  • The surface area is large enough to allow for sufficient absorption of nutrients and gases and secretion of waste products
  • The small volume means the diffusion distance to all organelles is short
• SA:V ratio decreases as cells get larger; this means that cells cannot grow bigger indefinitely; for larger cells the SA:V ratio is too small for cells to survive using only diffusion at the cell surface
  • Their metabolic requirements are higher
  • The surface area does not increase at the same rate as the metabolic requirements, so is not large enough too allow for a sufficiently high rate of exchange with the environment
  • The large volume means that the diffusion distance to the centre of the cell is long, so substances cannot diffuse quickly enough across the cell to reach the organelles where they are needed
• This means that once the SA:V ratio becomes too small, growth must stop and the cells must divide, giving rise to multicellular organisms
• Multicellular organisms have evolved adaptations to facilitate:
  • The exchange of substances between their internal tissues and the external environment, e.g.
    • Gas exchange systems
    • Digestive systems
  • Efficient transport of substances within their bodies
    • Circulatory systems

Surface area to volume ratio diagram

As the size of an organism increases, its surface area : volume ratio decreases; this means that as it gets larger, it becomes more difficult for an organism to gain enough oxygen and nutrients at its cell surface, as its requirements will increase faster than the available surface for diffusion

NOS: Students should recognise that models are simplified versions of complex systems

• Scientists use models to represent real world ideas, organisms, processes and systems that cannot be easily investigated
• Scientists can experiment on the models enabling them to test predictions and develop explanations for observations made
• The investigation below uses agar cubes to model the effect of changing surface-area-to-volume ratio on the rate of ion diffusion
• Although the cubes do not perfectly represent the shapes of real organisms, the scale factors and the resulting affect on diffusion still applies

Method

• Coloured agar is made up and cut into cubes of the required dimensions (eg. 0.5cm x 0.5cm x 0.5cm, 1cm x 1cm x 1cm and 2cm x 2cm x 2cm)
  • Purple agar can be created if it is made up with very dilute sodium hydroxide solution and Universal Indicator
  • Alternatively, the agar can be made up with Universal Indicator only
• The surface area, volume and surface area to volume ratio of these cubes is calculated and recorded
• The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
  • The same volume of dilute hydrochloric acid should be carefully measured out into each boiling tube
  • The acid should have higher molarity than the sodium hydroxide so that its diffusion can be monitored by a change in colour of the indicator in the agar blocks
• Measurements can be taken of either:
  • The time taken for the acid to completely change the colour of the indicator in the agar blocks
  • The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time (e.g. 5 minutes)

The steps used to investigate the effect of changing the surface area to volume ratio on diffusion

Analysis

• If the time taken for the acid to completely change the colour of the indicator in the agar blocks is recorded, these times can be converted to rates
• A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area : volume ratio of the agar cubes

To analyse the results of the investigation, calculate the rates of diffusion before drawing a graph for rate of diffusion against surface area : volume ratio